Various liquid dispensing systems have been developed for the precise application of a heated liquid onto a substrate. Dispensing systems for supplying liquid material in the form of filaments or other patterns are known in the art. These dispensing systems are conventionally used to apply thermoplastic materials, such as a hot melt adhesive, to various substrate materials during the manufacturing of diapers, sanitary napkins, surgical drapes, and other products. Typically, liquid material and pressurized process air are supplied to the dispensers where they are heated and distributed to one or more dispensing modules for application to the substrate. The heated liquid material is discharged from the dispensing module while heated pressurized process air is directed toward the dispensed liquid to attenuate or draw down the dispensed liquid material and to control the pattern of the liquid material as it is applied to the substrate.
Conventional liquid dispensing systems, shown schematically in FIG. 3, typically utilize a manifold for heating and distributing the pressurized air and liquid material to the dispensing modules. The manifold generally has a block configuration having a pair of opposed front and rear surfaces, a pair of opposed end surfaces, and opposed upper and lower surfaces. The manifold is configured to accommodate a number of dispensing modules that releasably couple to the manifold typically along the front surface. The dispensing module includes a liquid inlet and a process air inlet that communicate with a liquid outlet and process air outlet in the manifold. The dispensing module further includes a pneumatically or electrically actuated valve assembly for metering a precise quantity of the liquid and discharging the metered amount through a small-diameter dispensing orifice and onto a moving substrate positioned below the orifice. The dispensing end is generally adjacent the lower surface of the manifold. To increase liquid deposition control and accuracy, it is desirable to minimize the distance between the substrate and the dispensing end of the modules. As a result, the distance between the manifold and the substrate passing beneath the manifold is generally small. The distance between the lower surface of the manifold and substrate in conventional liquid dispensing systems, however, have some drawbacks.
One drawback is that the heaters in the manifold that heat the liquid and process air make the manifold, including the lower surface, very hot. This in turn heats the substrate as it passes underneath the manifold. The heating of the substrate may affect the thermal and structural properties of the substrate material, such as, for example, by weakening it. Moreover, heating the substrate may increase the curing time of the deposited liquid thereby affecting subsequent manufacturing steps, or may affect the spreading of the deposition pattern on the substrate, thereby depositing liquid where none is desired or possibly permitted, depending on the particular application.
Another drawback is that in some applications, such as when applying elastic strands onto a substrate, the angle at which the strands are fed toward the dispensing modules affects the coating of the strands as they pass by the dispensing orifice. In conventional dispensing systems, the manifold limits the angle at which the strands approach the dispensing orifice thus affecting coating efficiency of the strands. Yet another drawback is that servicing the substrate and the dispensing modules can be difficult in current dispensing systems. For instance, it can often be difficult to align or adjust the substrate, especially on that portion of the substrate directly beneath the manifold without contacting the heated manifold. Additionally, when servicing the dispensing modules a drip pan is typically used to drain the module so as to prevent any liquid from dripping onto the substrate. This may require that the dispensing modules be raised away from the substrate thereby disturbing the desired and established deposition height and deposition pattern.
A need therefore exists for an improved liquid material dispensing system which overcomes various drawbacks of prior dispensing systems, such as those described above.